Carbon nanostructures as thermal field emitters for waste heat recovery

Diamond & Related Materials 18 (2009) 563–566

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Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d

Carbon nanostructures as thermal field emitters for waste heat recovery Y.M. Wong a, W.P. Kang a,⁎, J.L. Davidson a, S. Raina b, J.H. Huang c a b c

Department of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, TN 37235, USA Interdisciplinary Program in Material Science, Vanderbilt University, Nashville, TN 37235, USA Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan, ROC

a r t i c l e

i n f o

Available online 27 November 2008 Keywords: Carbon nanotubes Energy conversion Nanodiamond Thermal-field emission

a b s t r a c t Traditional vacuum-based thermal energy conversion (VTEC) devices with “flat” metal emitters have been compromised by high working temperature mainly due to high work function and electron space charge effect. However, a VTEC system is inherently efficient due to unimpeded flow of electrons through vacuum and reduction of heat conduction by the vacuum medium. Consequently, VTEC devices are attractive in terms of space, weight and energy efficiency for waste heat recovery in a TEC system. Carbon nanostructured emitters including multiwalled carbon nanotubes (CNTs) and “ridge” nanodiamond thin films have shown relatively low threshold fields for electron field emission mainly due to their nanoscaled, high aspect ratio, emitting surfaces. In this study, the field-enhancing features of carbon nanostructures were investigated as thermal-field emitters for waste heat recovery. The turn-on field of CNTs was found to decrease from ~1.9 V/ μm at room temperature to ~ 0.9 V/μm at 400 °C. A high emission current of ~ 26 μA was achieved at relatively low field of ~ 1.5 V/μm and low temperature of 400 °C. The maximum efficiency of the VTEC device is estimated to be ~ 15% at 300 °C with a current density of 54 mA/cm2. © 2008 Elsevier B.V. All rights reserved.

1. Introduction High temperature fuel cells (HTFC) operated at N600 °C for the purposes of efficiency, and hydrocarbon sourcing simplification (e.g. direct use of JP8 feedstock) could benefit from being coupled with efficient thermal energy conversion (TEC) approaches. Such TEC system is being developed to potentially supplement the electrical output of compact HTFCs by tapping in to the heat generated. While the present “macro” HTFCs make use of the waste heat to, for instance drive generators, the compact TEC approach is expected to be more space, weight and energy efficient. A vacuum-based TEC (VTEC) system is inherently efficient due to unimpeded flow of electrons through vacuum and the reduction of heat conduction provided by the vacuum medium [1]. However, traditional VTEC devices with “flat” metal emitters are compromised by the high working temperature due to high work function and electron space charge effects [2]. Carbon nanostructured emitters including carbon nanotubes and nanodiamond thin films have shown relatively low threshold fields for electron field emission with typical turn-on fields ranging from 1–2 V/μm [3–5]. The low threshold fields of the nanocarbon emitters are mainly attributed to their high aspect ratio, i.e. large height to radius of curvature ratio, of the nanoscaled emitting surfaces. The field-enhancing features of the carbon nano-

⁎ Corresponding author. E-mail address: [email protected] (W.P. Kang). 0925-9635/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2008.11.018

structured emitters including “ridge” nanodiamond and carbon nanotubes (CNTs) are being investigated as technologically useful thermal-field emitters for VTEC devices in this study. 2. Experimental approach The “ridge” type nanodiamond and aligned tall multiwalled CNT thin films were grown on highly doped n-type silicon substrate by microwave plasma-enhanced chemical vapor deposition (MPCVD). The MPCVD system is equipped with 1.5 kW microwave power and a RF induction heater for substrate heating. The fabrication methods of the two samples have been described in details elsewhere [3,4]. The synthesis parameters of the samples are listed in Table 1. Briefly, CNTs were grown with a bi-metal layer of titanium, acting as a diffusion barrier layer and cobalt, the growth catalyst. The Si substrate of the “ridge” nanodiamond thin film was prepared for pre-nucleation by ultrasonication in nanodiamond powder suspension for 10 min. The morphologies of the carbon nanostructured samples were observed with S-4200 Hitachi scanning electron microscopy (SEM). The carbon bonding signatures were characterized by a Jobin Yvon Horiba Raman Spectrometer with a 632.81 nm He–Ne laser and power of 11 mW. A vacuum test chamber equipped with a 4″ boron-nitride (BN) heater controlled by a PID temperature controller was used for characterizing the thermal-field emission (TFE) characteristics of the VTEC devices, as shown schematically in Fig. 1(a). The vacuum system was evacuated to a base pressure of ~ 10− 8 Torr before performing TFE

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Table 1 MPCVD synthesis parameters of the aligned CNTs and the “ridge” nanodiamond thin film CNTs Microwave power (W) Temperature (°C) Pressure (Torr) Gas flow: H2/CH4/N2 (sccm) Time (min)

Ridge nanodiamond

Pretreatment

Growth

Growth

400 650 20 120/0/0 5

1000 650 20 120/15/5 10

1000 850 25 135/15/5 240

tests. A type K thermocouple placed on top of the sample was used for temperature measurement. The VTEC devices were tested in a diode configuration with a 1/2″ diameter flat “suspending” anode made of titanium without physical spacers between the cathode–anode gap, as shown by the schematic diagram of the test setup in Fig. 1(b). The planarity and the cathode–

Fig. 2. SEM micrographs of (a) the “ridge” nanodiamond thin film, and (b) the aligned CNTs (cross-sectional view). Insets are high magnification of the same.

anode gap were adjusted with an x–y position manipulator on the anode side. The device was biased negatively up to 2000 V with respect to the grounded anode. A 100 kΩ resistor was inserted at the anode side for measuring the voltage drop and hence the emission current collected, Fig. 1(b). 3. Results and discussion

Fig. 1. Schematic diagrams of (a) the vacuum test chamber, and (b) the TFE test setup.

The “ridge” nanodiamond film was found to have small ball-like clusters with size ranging from 2–4 μm, as shown in Fig. 2(a). The clusters are closely packed forming a continuous film but with gaps and voids between clusters. At high magnification, very sharp ridges on the surface having a thickness of only few nanometers and length up to micrometer long which resemble randomly oriented nanoscaled wedge emitters were observed. The ridges display random orientations with small nanodiamond grains on the walls of the ridges. Such nanostructures may be attributed to the simultaneous growth and etching of the nanodiamond film by the plasma at high power in the CVD process [4]. The CNT emitters have a height of ~65 μm, as shown by the cross-sectional SEM micrograph in Fig. 2(b). The tall CNTs are vertically aligned with diameter ranging from 10–20 nm, as observed under high magnification SEM. Raman spectra of the two carbon nanostructures is shown and compared in Fig. 3. The “ridge” nanodiamond film shows distinct peaks at 1335 cm− 1 and 1586 cm− 1, corresponding to sp3 diamond and

Y.M. Wong et al. / Diamond & Related Materials 18 (2009) 563–566

Fig. 3. Raman spectra of the as grown aligned multiwalled CNTs and the “ridge” nanodiamond thin film.

sp2 graphitic carbon, respectively. The two peaks are distinctively sharper with relatively smaller full-width-at-half-maximums (FWHMs) than those of the cauliflower-type nanodiamond [6]. The sharper sp3-diamond signature peak suggests higher diamond content in the layer. Further, a diffuse band near 1140 cm− 1 indicating presence of nanocrystalline phase in the diamond film is also observed [6]. The as grown CNTs also display two distinct peaks at 1323 cm− 1 and 1593 cm− 1, corresponding to the disorder-induced D-band and the stretching mode in the graphite plane (G-band), respectively. The D-band peak (defect mode) is generally attributed to carbonaceous particles and disorder-induced features such as defects in curved graphitic sheet and tube ends. The G-band peak (graphite-mode) indicates formation of graphitic sheet or graphene in the structure. The intensity ratio of the two first-order peaks, i.e. I(G)/I(D) is less than 1, suggesting poor crystallinity and high defects in the as grown CNTs [3]. The “ridge” nanodiamond film emitters were heated up to 400 °C and biased up to a voltage of 2000 V or a field of ~ 3 V/μm with a cathode–anode gap of ~650 μm. The device's TFE characteristics (I–V) are shown in Fig. 4. It is observed that the emission currents increase exponentially with respect to increasing applied voltage, suggesting

Fig. 4. TFE characteristics (I–V) of the “ridge” nanodiamond thin film. Inset: corresponding F–N plots of the emission data.

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stronger influence of field emission by field-enhancing nanostructures than thermionic emission. The turn-on field (defined as the applied field required for obtaining an emission current of 100 nA) was found to decrease from N6.0 V/μm at room temperature to 1.2 V/μm at 400 °C. An emission current of 10.6 μA was achieved at ~2.1 V/μm and 400 °C. The TFE test results of the tall CNTs are shown in Fig. 5. It is observed that the emission currents also increase in an exponential fashion with respect to increasing applied voltage. Further, the turn-on field was found to have decreased from 1.9 V/μm at room temperature to 0.9 V/ μm at 400 °C with a cathode–anode gap of ~370 μm. A high emission current of 26.0 μA was achieved at ~540 V or 1.5 V/μm at 400 °C. The corresponding Fowler–Nordheim plots [7,8], i.e. Ln(I/V2) vs. 1/V of the emission data of the two samples are shown in Fig. 4 (inset) and Fig. 5 (inset). The data conforms to F–N theory with linear relationship for applied fields less than 3.0 V/μm in this study. Assuming the geometrical aspect ratio of the nanostructures remained unchanged, the decreasing slope of the F–N curves for increasing temperature suggests a reduction in the effective work-function of the nanodiamond film and/or an increase in the emission area at the elevated temperature. High current performance of the CNT emitters was also characterized at 300 °C. It was observed that one of the emitters achieved a high emission current of 3.8 mA or a current density of 54 mA/cm2 at a relatively low applied field of 1.7 V/μm. The primary heat loss mechanisms of a VTEC device are thermal radiation losses, thermal backflow, and resistive heating and heat conduction losses via electrical connection [2,9]. The efficiency of a field-enhanced VTEC in a diode configuration is difficult to compute. However, the thermal emission at elevated temperature can be extracted by subtracting the field-enhanced emission at room temperature. The thermal radiation power losses, Pr of a VTEC can be expressed as [9]: 2


4 4 3 σ T −T Pr = 4  H C 5 1 1 eH + eC −1

ð1Þ

where σ = 5.67 × 10− 12 (W/cm2 K4) is the Stefan–Boltzmann constant, TH and TC are the absolute temperature of the hot emitters and the cold collector, and εH and εC are the emissivity of the respective

Fig. 5. TFE characteristics (I–V) of the tall multiwalled CNTs. Inset: corresponding F–N plots of the emission data.

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By maximizing η with respect to Rl, i.e. dð1=ηÞ = 0, the maximum dRl efficiency can be further simplified as [9]: η=

1 1+C

where C =

Fig. 6. Projected thermal efficiency, η and output power density vs. temperature of the VTEC device. Theoretical Carnot thermal efficiency is also included for comparison purposes.

ð7Þ ePr kB TH J .

For the carbon nanostructured emitters operating at a current density of 54 mA/cm2 and a biased field of 1.7 V/μm at 300 °C, Pr is found be ~ 0.015 W/cm2, and Г = 5.69. Consequently, the maximum efficiency of the VTEC device is estimated to be ~ 15%, which is comparable to commercial Si photovoltaic cells. It is worth to note that a great deal of assumptions has been made to arrive at the estimation of the VTEC's maximum efficiency. Greater efficiency is also expected to be achievable by operating the VTEC device at higher temperature of 600–800 °C. Assuming current density changes linearly with increasing temperature, the projections of η and the corresponding power density of the VTEC device is shown in Fig. 6. Theoretical Carnot thermal efficiency is also included in the plot for comparison purposes. According to the projection, the VTEC device is expected to achieve a thermal efficiency of ~22% and a power density of ~ 0.7 W/ cm2 at 800 °C. 4. Conclusions

material. Further, the resistive heating (I2R), Pj and heat conduction losses, Pk per unit emitter area are given by [9]: 

 1 ð JAE Þ2 Rl AE

Pj =

Pk =

ð2Þ

   kl Al TH −TL AE L

ð3Þ

where Rl, kl, Al, L are respectively the electrical resistance, thermal conductivity, cross-sectional area, and length of the lead wire, AE is the area of the emitter, J is the net current density and TL is the load temperature. The thermal efficiency of the VTE, η can be formulated as [9]: η=

PL Pe + Pr + Pk + Pj

ð4Þ

where PL = JΔVL is the useful load power per unit emitter area. Pe is the sum of potential energy imparted to electrons and their average kinetic energy at emitter temperature, and can be expressed as [9]:     2kB TH 2kB TH = J ΔVL + ΔVl + ΔVA + Pe = J ΔVE + e e

ð5Þ

where ΔVE is the effective emitter work function, kB is the Boltzmann constant, e is the electron charge, ΔVl = JAERl is the voltage drop through the lead, and ΔVA is the effective anode potential. Consequently, η=  J ΔVL + JAE Rl + ΔVA +

2kB TH e

JΔVL  + Pr +

π2 6

 2 kB e

ðTH2 −TL2 Þ 1 AE

Rl

− 12 J 2 AE Rl

ð6Þ

We have synthesized carbon nanostructures as thermal field emitters including “ridge” nanodiamond and multiwalled CNTs grown by MPCVD on Si substrate. The TFE characteristics of the nanocarbon emitters were investigated. The turn-on field of the “ridge” nanodiamond decreased from ~ 6.0 V/μm at room temperature to 1.2 V/μm at 400 °C. While the turn-on field of the tall multiwalled CNTs was found to decrease from 1.9 V/μm at room temperature to 0.9 V/μm at 400 °C. Further, a high emission current of 26 μA was achieved at ~ 1.5 V/μm at 400 °C. The data conforms to the F–N field emission theory for applied fields less than 3 V/μm. The maximum efficiency of the VTEC device is estimated to be ~ 15% at 300 °C with a current density of 54 mA/cm2, comparable to commercial Si photovoltaic devices. Overall, carbonbased nanostructures synthesized by MPCVD show promising results with high emission current at low temperature and low applied electric field for waste heat recovery in TEC applications. References [1] S.H. Shin, T.S. Fisher, D.G. Walker, A.M. Strauss, W.P. Kang, J.L. Davidson, J. Vac. Sc. Techn. B 21 (2003) 587. [2] G.N. Hatsopoulos, E.P. Gyftopoulos, Thermionic Energy Conversion, MIT Press, Cambridge, 1973. [3] Y.M. Wong, W.P. Kang, J.L. Davidson, B.K. Choi, W. Hofmeister, J.H. Huang, Diamond Relat. Mater. 14 (2005) 2078. [4] S. Raina, W.P. Kang, J.L. Davidson, Diamond Relat. Mater. 17 (2008) 790. [5] K. Subramanian, W.P. Kang, J.L. Davidson, W.H. Hofmeister, B.K. Choi, M. Howell, Diamond Relat. Mater. 14 (2005) 2099. [6] K. Subramanian, W.P. Kang, J.L. Davidson, W.H. Hofmeister, Diamond Relat. Mater. 14 (2005) 404. [7] R.H. Fowler, L.W. Nordheim, Proc. R. Soc. (London) 119 (1928) 173. [8] L.W. Nordheim, Proc. R. Soc. (London) A121 (1928) 626. [9] M.M. El-Wakil, Nuclear Energy Conversion, American Nuclear Society, Illinois, 1978.